1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to an image sensor with a 3D stack structure, in which pixels of a top plate are realized as image pixels and pixels of a bottom plate are realized as pixels for realizing a phase difference AF, so that the phase difference AF can be realized without loss of resolution.
2. Description of the Related Art
Recently, so-called electronic cameras are widely spread. The electronic camera operates through a process in which when an object is captured through an optical system, an image is formed on an image sensor that is a semiconductor imaging device, the image of the object is converted into an electrical signal, and image data of the obtained still image is recorded on a recording medium such as a semiconductor memory or a magnetic disk.
Most electronic cameras are provided with an auto-focus (hereinafter, referred to as ‘AF’) device that controls capturing conditions in order to automatically focus an image of an object. Among conventional methods for obtaining such auto-focusing, a technology called a phase difference AF is widely used.
In the phase difference AF, since it is possible to immediately recognize a distance between an in-focus point and a film surface based on an inter-image distance (a phase difference) when light from an image of an object is received in a phase difference detection sensor provided with a CCD line sensor, it is mainly used in that it is possible to allow the in-focus point to coincide with the film surface through one-time driving.
In a structure of a conventional imaging surface phase difference AF, a specific number of pixels in a pixel array of a sensor are artificially blocked or a photodiode is allowed to move, so that phase information is obtained.
The structure of the imaging surface phase difference AF includes a first element having a pixel function for realizing an image and a second element for realizing a phase difference AF function. In this case, the second element is obtained by performing special processing for the first element and is provided as a plurality of pairs.
Due to such a structure, even though there is no separate phase difference AF sensor module, since it is possible to realize the phase difference AF function through only a sensor itself, it is possible to use a fast phase difference AF in compact digital cameras.
However, when the phase difference AF is realized on the sensor surface, since pixels to be used for forming an image are lost, overall resolution is reduced. That is, in the structure of the conventional imaging surface phase difference AF, there is a problem that in order to improve resolution, it is difficult to realize the phase difference AF, and in order to realize precise phase difference AF, resolution is reduced.